Co-inoculation effect of rhizospheric microorganisms viz. N2 fixer (Azotobacter chroococcum), PSB (Bacillus megaterium and B. polymyxa) and  PGPR (Pseudomonas fluorescens and Bacillus sp.) on occupancy of inoculated Rhizobium sp. in root nodules of urdbean were examined in two separate studies under field conditions at Pantnagar. Inoculated Rhizobium sp. alone formed 23 to 58 % nodules in competition with native Rhizobium at different crop growth stages. Inoculation of A. chroococcum, B. megaterium and P. fluorescens with Rhizobium sp. significantly increased the percent nodule occupancy of inoculated Rhizobium sp., by 13.0 to 38.3, 6.2 to 52.9  and 6.2 19.6 % over Rhizobium sp. alone at different intervals, respectively. Bacillus sp. and B. polymyxa indicated adverse effect on competitive ability of inoculum Rhizobium sp..  Combined inoculation of Rhizobium sp. + A. chroococcum  + Bacillus sp. and Rhizobium sp. + B. megaterium + P. fluorescensb gave  significantly more  nodule occupancy, by 14.6 to 54.3 and 11.8 to 66.6 %,  and recorded better nodule number and nodule dry weight than Rhizobium sp. alone at different intervals.

Keywords : Urdbean; Vigna Mungo. Rhozobium; PSB; Rhizobacteria; Strain Competition.